Refractive index (n), also known as the index of Refraction, is defined as the ratio of the speed of light in a vacuum to the speed of light in a material and is dimensionless quantity shown by n:n=velocity of light in the vacuum/velocity of light in the substance.
In other words, when a light beam passes form one substance (air) to another (a liquid), it is bent or refracted because of the difference in speed between the two substances. The refraction index indicates the degree of this refraction. Additional, refractive index is measures as the sine of the angle of bending (deflection) of light as it passes from one medium to another.
Refractive index is a property of optical materials that determines how fast light travels through it. The numerical n value indicates the light bending power of a medium such as a chemical. The greater the bending power, the greater the refractive index. In a medium, the speed of light depends on the wavelength and temperature. For this reason refractive index is usually measured and reported at 20° C. with the D line sodium light.
The refractive index is a thermodynamic property and is a state function, which for a pure fluid depends on temperature and pressure. Since the velocity of light in a fluid is less than the velocity of light in a vacuum, its value of a fluid is greater than unity. Liquids have higher values of refractive index than that of gases. For gases the values of refractive index are very close to unity. The refractive index or refractivity (n) can be easily measured by the sodium D line of a simple refractometer at a temperature of interest. Values of n at 20 and 25° C. are given by the API Technical Data Book for many different hydrocarbons.
All frequencies of electromagnet radiation (light) travel in the same speed in vacuum (2.998×108 m/s); however, in a substance the velocity of light depends on the nature of the substance (molecular structure) as well as the frequency of the light. For this reason, standard values of refractive index must be measured at a standard frequency. Usually the refractive index of hydrocarbons is measured by the sodium D line at 20° C. and 1 atm. In some references the values of refractive index are reported at 25° C.; however, the refractive index is usually measured at 20° C. and 1 atm, and is usually used as a characterization paramount for hydrocarbons and petroleum factions.
Refractive index testing procedure is used to determine the quality of every essential oil. Light behaves differently depending upon the density of the material it is passing through. The reading is compared to established literature; deviations are indicative of adulteration. The index of air is 1.00 and all indices are referred to the index of air, i.e. the index of water being 1.33 means that the speed of light in air is 1.33 times greater as the speed of light in water. Ice refractive index of 1.31, while air has a refractive index of 1.000277. Refractive indexes of hydrocarbons vary from 1.3 for propane to 1.6 for some aromatics; however, aromatics have refractive index value greater than napthenes, which in turn have refractive indexes greater than paraffins.
Refractive index (n) is a useful parameter to characterize hydrocarbon systems and is needed to estimate the composition of undefined petroleum factions. For example, the refractive index at some reference conditions (i.e., 20° C. and 1 atm) is a useful characterization parameter to estimate the composition and quality of petroleum factions. It is also used to estimate many physical properties such as molecular weight, equation of state paraments, the critical constants, or transport properties of hydrocarbon systems.
For pure liquids and mixtures, refractive index is a bulk property that can be easily and accurately measured by an optical instrument called refractometer. Certain types of refractometers can be used for measuring gases, liquids, and even transparent or translucent solids such as gemstones.
Refractive index can be measured by digital refractometers with a precision of 0.0001 and temperature precision of 0.1° C. The amount of sample required to measure refractive index is very small and ASTM D1218 provides a test method for clear hydrocarbons with values of reflective indexes in the range of 1.33-1.5 and the temperature range of 20-30° C. In the ASTM D1218 test method the Baush and Lomb refractometers is used. Refractive index of viscous oils with values up to 1.6 can be measured by the ASTM D1747 test method. Samples must have clear color to measure their refractive index; however, for darker and more viscous sample in which actual refractive vale is outside the range of application of refractometer, samples can be diluted by a light solvent and refractive index of the solutions should be measured. From the composition of the solution and refractive index of pure solvent and that or the solution, refractive index of viscous samples can be determined. A model Abbe refractometer (Leica), for example, measure refractive index of liquids within the temperature range of −20 to 100° C. with temperature accuracy of ±0.01° C. Because of simplicity and importance of refractive index it would be extremely useful if laboratories measure and report its value at 20° C. for a petroleum product, especially if the composition of the mixture is not reported.
There are four main types of fluid refractometers: traditional handheld refractometers, digital handheld refractometers, Abbe refractometers, and inline process refractometers.
Ad traditional handheld refractometer is a handheld analog instrument for measuring refractive index that works on the critical angle principle. They utilize lenses and prism to project a shadow line onto a small glass reticule inside the instrument, which is then viewed by the user though a magnifying eyepiece. N use, a sample is sandwiched between a measuring prism and a small cover plate. Light traveling through the sample is either passed through to the reticule or totally internally reflected. The net effect is that a shadow line is formed between the illuminated area and the dark area. It is at the point that this shadow line crosses the scale that a reading is taken. Because refractive index is very temperature dependent, it is important to use a refractometer with automatic temperature compensation. Compensation is accomplished through the use of a sample bi-metal strip that moves a lens or prism in response to temperature changes.
In optics, a digital handheld refractometer is an instrument for measuring the refractive index of materials. Most operate on the same general critical angel principle as a traditional handheld refractometer. The difference is that light for an LESD light sources is focused on the underside or prism element. When a liquid sample is applied to the measuring surface of the prism, some of the light is transmitted through the solution and lost; while the remaining light is reflected onto a linear array of photodiodes creating a shadow line. The refractive index is directly related to the position of the shadow line on the photodiodes. The more elements there are in the photodiode array, the more precise the readings will be, and the easier it will be to obtain readings for emulsions and other difficult-to-read fluids that from fuzzy shadow lines. Once the position of the shadow line has been automatically determined by the instrument, the internal software will correlate the position to refractive index, or to another unite of measure related to refractive index, and display a digital readout on an LCD or LED scale.
Digital handheld refractometers are generally more precise than traditional handheld refractometers, but less precise than most benchtop refractometers. Then also may require a slightly larger amount of sample to read from (since the sample is not spread thinly against the prism. Nearly all digital refractometers feature automatic temperature compensation (for Brix at least). Like most forms of electronics, this type of unit is always getting smaller and more ergonomic.
Am Abbe or laboratory refractometer is a bench-top refractometer that offers the highest precision of the different types of refractometers. Nearly one and a half century after their introduction, refractometers have come a long way in terms of usefulness, though their principle of operation has changed very little.
Ernst Abbe, working for the Zeiss Company in Jena, Germany in the late 1800s, was the first to develop a laboratory refractometer. These first instruments had built-in thermometers and required circulating water to control instrument and fluid temperatures. They also had adjustments for eliminating the effect of dispersions. These first instruments had analog scales from which the readings were taken.
There have been many refinements regarding teas of use and precision to these instruments over the decades, but they still operate on the same principle. They are still used today as an inexpensive alternative to digital laboratory refractometers. They are also possibly the easiest method to find the refractive index of solid samples, such as glass, plastics, and polymer films. Some Abbe refractometers utilize a digital display for the measurement, to eliminate the need for discerning between small graduations. The user still has to adjust the view to obtain the reading, however.
The first truly digital laboratory refractometers began appearing in the late 197s and early 19802, and no longer depended on the user's eye to determine the reading. They still required the use of circulating water baths to control instrument and fluid temperature. They did, however, have the ability to electronically compensate for the temperature differences of many laboratory refractometers, while much more accurate and versatile than thief analog Abbe counterparts, are not capable of reading solid samples.
IN the late 1990s, Abbe refractometers with the capability to read at wavelengths other than the standard 589 nanometers became availability. These instruments utilize special filters of the desired wavelength of light, well into the near infrared (though a special viewer is required to see the infrared rays). Multi-wavelength Abbe refractometers can be used to very easily determine a sample's Abbe number.
The most advanced instruments of today use solid-state Peltier effect devices to heat and cool the instrument and the sample, eliminating the dependence on an external water bath. The software on most of the current instruments is now very advanced and offers features such as programmable user-defined scales and a history function that recalls the last several measurements. Several manufacturers provide easily usable controls, with the capability to operate from and export readings to a linked computer.
Previously refractive index readings from manual refractometers were obtained by visual inspection. A sample is placed in the refractometer and a knob that moved a graduated scale is rotated until two lines representing light refection through the material and space or air are aligned then the meter reading is recorded using visual inspection. Currently lab-scale automatic refractometers are being used which output the numeral value of the refractive index using a digital display.
Inline process refractometers are a type of refractometer designed for the continuous measurement of a fluid flowing through a pipe or inside a tank throughout the manufacturing process. These refractometers typically consist of a sensor, placed inline with the fluid flow, couple to a control box. The control box usually provides a digital readout as well as 4-20 mA analog outputs and relay outputs for controlling pumps and valves.
Refractometers are widely used in oil industry, fat industry, pharmaceutical factories, paint, and food processing, among others. A refractometer can be used to determine the identity of a n unknown substance based on its refractive index, to assess the purity of a particular substance, or to determine the concentration of one substance dissolved in another. Most commonly, refractometers are used for measuring fluid concentrations such as the sugar content ((brix level) of fruits, vegetables, juices and carbonated beverages, or of cutting fluids, urine specific gravity, blood protein concentration, salinity, antifreeze, industrial fluids, etc. Materials measured can be chemicals, syrups, Uren, food, pharmaceuticals petroleum products and the like. For testing refractive index, honey, coolants, specific gravity in urine, etc. Measure soluble solids (BRIX) percentage in fruit, juices, cooking oils and other various solutions. What current refractometers cant do is directly measure and display the thermophysical properties of the material being tested.